1. Field of the Invention
The present invention relates to novel crosslinkable hot-melt adhesive compositions including a prepolymer containing free isocyanate functional groups which comprises the product of reaction between an ethylene/vinyl acetate copolymer containing hydroxyl functional groups and a polyisocyanate.
The compositions of this invention, typically combined with an adhesive (tackifying) resin, enable production of adhesive bonds which withstand elevated temperatures after crosslinking of the free NCO functional groups, usually under the influence of atmospheric moisture.
2. Description of the Prior Art
The adhesives known to this art are generally in liquid form and become rigid after application, while ensuring the bonding of the surfaces to be united. This stiffening usually results from the polymerization or polycondensation of the basic components of the adhesive.
For a number of years, ever increasing attempts have been made to provide adhesive compositions in solid form as well. These include the hot-melt adhesives which are based on a thermoplastic resin, solid at room temperature, possessing adhesive properties. Such adhesives become fluid when heated, the bonding of the surfaces to be united being ensured when the adhesive again becomes rigid on cooling. These hot-melt adhesives are generally prepared by the combination of two basic constituents: a thermoplastic resin and an adhesive (tackifying) resin, with which there may be combined additives such as waxes, stabilizers, fillers, plasticizers, and others. The most widely used basic thermoplastic resins are the polyamides, atactic polypropylene and, in particular, ethylene/vinyl acetate (EVA) copolymers. In the present state of this art, these hot-melt adhesives have good adhesive properties, but have the disadvantage of exhibiting poor heat strength, a strength which barely exceeds 70.degree. to 80.degree. C.
To overcome this disadvantage, a new generation of crosslinkable hot-melt products has been developed: crosslinkable polyurethane hot-melt adhesives. These adhesives are prepared in conventional manner by reaction of a polyisocyanate with polyols of polyester type, at least one of which is solid at room temperature. Such adhesives are applied at high temperature in the molten state. They have the disadvantage of being incompatible with the adhesive (tackifying) resins, which greatly restricts their formulation options. This is then reflected in a limited capacity for adhesive bonding to certain well-defined supports and substrates. In addition, these products have "open times" on the order of a few minutes, and this is too long for certain high-rate applications. The "open time" of a hot-melt adhesive is the time available to effect the adhesive bonding, between the time when the adhesive in the molten state is applied to the first support to be bonded and the time when the hot-melt adhesive is no longer sufficiently fluid to permit the second support to be properly assembled.
Another type of crosslinkable hot-melt adhesive has been developed from hydroxylated ethylene/vinyl acetate copolymers reacted with a blocked polyisocyanate in stoichiometric amount, as described in JP 108,484 or European Application 88/401,259. These particular hot-melt adhesives, which are compatible with .the adhesive (tackifying) resins, have good adhesive properties, but present the disadvantage of requiring heat treatments at temperatures of at least 120.degree. C. for several minutes to ensure that they are well crosslinked. Such adhesive bonding conditions are not acceptable in the case of certain thermally sensitive supports.